docs: Add more design polish (#43802)

Fixing the theme toggle popover, search results design, and
sidebar/table of contents responsiveness.

Release Notes:

- N/A

docs/theme/css/general.css

@@ -68,6 +68,7 @@ h3,
 h4,
 h5,
 h6 {
+  position: relative;
   font-family: var(--title-font);
   font-weight: 480;
   color: var(--title-color);
@@ -152,10 +153,9 @@ h3:target::before,
 h4:target::before,
 h5:target::before,
 h6:target::before {
-  display: inline-block;
   content: "»";
-  margin-inline-start: -30px;
-  width: 30px;
+  position: absolute;
+  left: -1.5ch;
 }
 
 hr {